The present invention relates to adaptive filters and, in particular, to methods and apparatus for determining filter coefficients for adaptive filters.
Data communications receivers often employ adaptive filter systems to compensate for distortion, such as inter-symbol interference (“ISI”), introduced into the data signal by the communication channel. Such systems often employ an adjustable filter including a tapped delay line where the signals appearing at the taps are scaled by a set of filter coefficients or “tap weights” and then summed. Such filters attempt to synthesize an inverse function H−1(z) of the communication channel transfer function H(z) by appropriate selection of the filter coefficients of the adjustable filter.
Various techniques have been used to update the filter coefficients of adaptive filters, including zero-forcing processes and least-mean-square (“LMS”) processes. These processes can be challenging to apply at high speeds, such as the multi-gigabit-per-second data rates that may occur in optical data communication systems. Zero-forcing processes require a history of detected symbols, and LMS processes require a history of input samples. These histories can be difficult to collect and maintain at high speeds. Moreover, these techniques require the generation of an error signal, which is correlated to the input in LMS processes and is correlated to the decision history in zero-forcing processes. These correlations can be difficult to maintain at high speeds.